Exploring the Skull Vibration Induced Nystagmus Test (SVINT)

We’re excited to share an assessment technique that has significantly enhanced our ability to evaluate patients with vestibular hypofunction: the Skull Vibration Induced Nystagmus Test (SVINT). After hearing about it a few times over the past few years, we decided to investigate further and are glad we did. Given our access to infrared goggles, implementing SVINT was quite affordable since portable handheld massage devices can be picked up at places like Walmart.

Clinically, the test has helped us pick up vestibular hypofunctions when our other battery of tests such as looking for gaze evoked nystagmus, head impulse test, head shaking nystagmus test, and dynamic visual acuity tests were unremarkable. 

Getting the Right Equipment

The main challenge was finding a massage device that vibrated at 100Hz. Most handheld vibrators operate at lower frequencies and typically specify their speed in RPM rather than Hz. To convert RPM to Hz, we used the simple conversion where 1Hz equals 60RPM. Although purchasing a device from Walmart doesn’t guarantee precise calibration, it’s close enough for our purposes. Dumas in 2017 also indicated that a frequency range of 60-120Hz was sufficient for testing for vestibular loss, with 100Hz being ideal. Personally, we opted for a corded model, following Timothy Hain’s recommendation, to avoid performance issues due to low battery levels in cordless devices. Just to warn you, if you look to buy a hand held vibrator on Amazon, devices will pop up that you were not looking for. 

Initial Results and Experiences

When we first used our vibrator, we were pleasantly surprised at how effectively it elicited nystagmus in patients with a vestibular hypofunction. We have yet to test it on someone with a semicircular canal dehiscence, as we have not seen someone with that condition for at least a couple of years. Of note, Dumas in 2024 recommends having a vibrator with a 400Hz capacity for this patient population.

Performing the Test

Instructions:

1. Positioning: Stand beside or behind the patient, holding the vibrator in your dominant hand for consistency.
2. Application: The vibrator is held firmly and applied perpendicularly to the skin over the mastoid process, just behind the auricle at the level of the external acoustic meatus. Avoid stimulating the tip of the mastoid process to prevent activating proprioceptive afferents from the trapezius and sternocleidomastoid muscles. Stimulation though the vertex can also be used and may be more sensitive if testing for a superior semicircular canal dehiscence
3. Pressure: Apply a pressure of about 10 N (2.2lbs of force).
4. Stimulation Trials: Perform three trials on each mastoid, each lasting about 5–10 seconds at 100Hz.
5. Visual Observation: Use Frenzel or infrared goggles to observe eye movements, ensuring the test is conducted without any visual fixation.

You can check out our YouTube video (8min) demonstrating how to perform the test and what a positive test looks like.

Validity Criteria:

• Nystagmus should start with stimulation, stop with its withdrawal, and not present any secondary reversal.
• It should be sustained, reproducible, and consistent in direction.
• The nystagmus beats in the same direction regardless of which mastoid is stimulated.

SVINT sensitivity and specificity:

• Sensitivity: 98% for total unilateral vestibular loss (UVL), 75% for partial UVL, and 30% for brainstem lesions.
• Specificity: 94%.

Safety Precautions

Before performing SVINT, consider the following patient populations:

• Recently operated otosclerosis
• Retinal detachment
• Recent cerebral hematoma
• Poorly controlled anticoagulant therapy

Understanding Why the Horizontal Nystagmus

• Cancelation Principle: Eye movements induced by simultaneous anterior and posterior canal activation in one labyrinth are opposite and cancel each other out, leaving just the horizontal component.
• Unilateral Vestibular Loss (UVL): In patients with left UVL, 100Hz stimulation will activate neurons in the healthy right labyrinth. The resulting nystagmus will beat away from the affected side.
• Semicircular Canal Dehiscence (SCD): In patients with left anterior semicircular canal dehiscence, the stimulation will strongly activate afferents from the left side, causing nystagmus to beat toward the affected side.

Summary and Further Reading

The Skull Vibration Induced Nystagmus Test is a valuable bedside test for identifying vestibular hypofunction and semicircular canal dehiscence. Its implementation has proven beneficial in our clinical practice and has become a "go to" test for vestibular loss.

We hope you found this information helpful and that it enhances your clinical assessments. For further details, check out the references below for more in-depth understanding.

Resources:

1. The Skull vibration-induced
   Nystagmus Test of vestibular
   Function—A Review
2. Skull Vibration-Induced Nystagmus in Superior Semicircular Canal Dehiscence: A New Insight into Vestibular Exploration—A Review
3.  Sound, Vibration, and the Vestibular System with Ian Curthoys - YouTube video from Balancing Act Rehab.